Calcium movements during E-C coupling in giant muscle fibers from the barnacle will be followed using the aequorin technique. Entry of extracellular calcium during the EDTA-induced calcium spike will be determined by simultaneously measuring the calcium current with a voltage-clamp and calcium ion entry with aequorin. Normal release of intracellular calcium from the SR in response to voltage clamped changes in membrane potential will be determined by simultaneously measuring membrane potential, membrane current, calcium transient, and tension from fibers which have not been EDTA-injected. These experiments will determine the voltage dependent mechanism of calcium release. Calcium movements during re-accumulation by the SR will be determined, first by analyzing the effects of known SR inhibitors on the time course of the falling phase of the calcium transient, and second by using myothermic measurements to unmask the time course of the SR pump activity. To meet and overcome the limitations of the aequorin technique, two new methods of monitoring the intracellular calcium concentration are proposed. The first is an ultra-high frequency (100KHz.) dual-beam spectrophotometer that is based on an elegant new light modulator coupled to a high signal-to-noise de-modulator, and which will be used in conjunction with calcium-sensitive dyes. The second is the isolation, extraction, and purification of a calcium-sensitive photoprotein having either faster kinetics or simpler stoichiometry than the aequorin-calcium reacton.